.bazelrc: switch over to PY3 (rules_docker is now fully PY3 compliant)

Change-Id: I53edb8eae81779d5b8cea36e3bec4c05ca2c6e0d
diff --git a/.bazelrc b/.bazelrc
index 419641e..dc92a13 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -1,7 +1,7 @@
 # https://github.com/bazelbuild/rules_docker/issues/842
-build --host_force_python=PY2
-test --host_force_python=PY2
-run --host_force_python=PY2
+build --host_force_python=PY3
+test --host_force_python=PY3
+run --host_force_python=PY3
 build --stamp
 build --workspace_status_command=./bzl/workspace-status.sh
 test --build_tests_only